Surface phase transitions in CO chemisorption on Pt{110}

Abstract

Abstract The chemisorption of CO on Pt{110} results in various coverage dependent phases. These have been investigated both during adsorption at 300 K and during desorption by means of continuous monitoring of LEED beam intensities, the CO derived levels in ARUPS, and thermal desorption spectra. We find that at very low coverages, isolated species on the reconstructed Pt{110}−(1 × 2) surface coexist with islands of CO on Pt{110}−(1 × 1). At higher coverages (⩾0.5), when the (1 × 2)→(1 × 1) transition is completed, repulsive interactions between nearest neighbour CO molecules result in a second state in the desorption spectra. Defect sites due to imperfect lifting of the surface reconstruction by CO inhibit the formation of the ordered phase at 300 K as the coverage approaches 1. Once these imperfections have been removed, a well ordered phase, designated (2 × 1) p1g1, can be formed. The “(2 × 1)” → (1 × 1) order-disorder transition is reversible. The CO axis is tilted away from the surface normal within the islands and the pigl phase. At all coverages CO is adsorbed into “on-top” sites as inferred from HREELS.